The present invention relates to an ordinary zoom lens system capable of close-up photography.
Many zoom lens optical systems capable of close-up photography comprising focusing lenses, correcting lenses, and fixed lenses are known in the art. In one of these known zoom lens systems, either of a group of variable power lenses and a group of correcting lenses is fixed but the other lens group is moved so that a certain relationship of between the two lens groups maintained in an ordinary zoom lens optical system is changed in attaining the state capable of close-up photography.
According to another known method, the lens spacing between variable power lenses and correcting lenses in an ordinary zoom lens system is changed in advance and when zooming is performed the picture point is shifted, whereby a state capable of close-up photography is attained.
In performing such operations in the zoom optical system, in order to change a relationship of the spacing between variable power lenses and correcting lenses which is maintained in case of ordinary photography, a cam cylinder having a cam groove formed therein for moving the two groups of lenses is divided into two portions, or the length of the cam groove is extended beyond the ordinary movement range. However, such arrangements involves various technical problems and difficulties. For example, the precision is reduced by division of the cam cylinder into two portions, and an additional space must be considered for extending the cam groove and designing becomes difficult because of such extension of the cam groove.
It is therefore a primary object of the present invention to provide a zoom lens system in which the foregoing defects involved in the conventional techniques are eliminated, by providing a zoom lens system in which in the case of close-up photography, as well as in the case of ordinary photography, variable power lenses and correcting lenses can be moved without changing a spacing maintained between the two lens groups in ordinary photography, and in which these two lens groups can be integrally moved by a prescribed length toward an object to be photographed with respect to focusing lenses and fixed lenses while keeping a constant spacing maintained between the two lens groups, and wherein close-up photography is made possible by the zooming operation used in ordinary photography.
The present invention will now be described in detail by reference to embodiments illustrated in the accompanying drawings.